Volumetric flow sensors which are based on the measurement of the differential pressure across a restriction are widely used in the measurement of fluid flow, cf. the journal "Chemical Engineering", May 1996, pages 94 to 102. They have an orifice plate which has a standard thickness and a bore of standard diameter, and which is fixed fluid-tight between a first pipe and a second pipe conducting a fluid to be measured. This is generally done by inserting the orifice plate between two flanges, one provided at the end of the first pipe and the other provided at the end of the second pipe facing said one flange. Further details are explained below in connection with FIG. 24.
The diameter of the orifice is smaller than the inside diameter of the pipes. A first pressure sensing element for sensing the fluid pressure upstream of the orifice plate is operatively connected with a bore formed in the first pipe close to the orifice plate. A second pressure sensing element for sensing the fluid pressure downstream of the orifice plate is operatively connected with a bore formed in the second pipe close to the orifice plate.
Frequently, the bore of the first pipe is provided in the flange associated with this pipe, and the bore of the second pipe is provided in the flange associated with this second pipe.
Compact versions in which the bores for sensing the pressure, the pressure sensors, and associated evaluation electronics with a housing are united are also in common use.
Since the orifice has a diameter smaller than the inside diameter of the two pipes, thus forming a restriction in the flow channel, according to Bernoulli's law, a pressure differential is produced across it by a moving fluid. From this pressure differential, volumetric flow rate can be determined, which is proportional to the square root of the quotient obtained by dividing the pressure difference by the density of the fluid.